The traditional aerobic wastewater treatment system produces a stream of treated water and a volume of excess organic biologic waste sludge in some proportion to the mass of organic material removed from the wastewater treated. This biological waste activated sludge is then often treated further to reduce its volatile organic content and volume by removal of the water that is released during this further treatment before final removal from the treatment plant for final disposal. The content, tonnage and volume of the excess sludge are major cost factors in the final disposal of the treated waste solids which is usually by land application, landfill, incineration or possibly by commercial sale as fertilizer.
The aerobic digestion of biological residuals from excess waste activated sludge is a commonly used method for reducing the amount of solids that must be finally disposed of from the wastewater system. This is usually accomplished by aerating and mixing the residual solids in a digester holding tank. Excess activated sludge generated by the activated sludge wastewater treatment system is regularly removed from the treatment process and placed in the digester tank. When the digester tank is full the aeration and mixing is stopped, the solids are allowed to settle out to the bottom and the water removed by a process called supernation. This separation may take one or more days during which time the aerobic digestion process is stopped since the aeration must be shut down to allow settling out of the solids. Stoppage of the aeration deprives the microorganisms, responsible for the digestion process, of oxygen needed for respiration and metabolization of the volatile organics in the digester.
A significant portion of the aerobic organisms in the sludge that do the digestion of the organic solids cease metabolic activity and facultative organisms in the solution convert their respiration to their anaerobic mode during the settling out, delaying resumption of the aerobic digestion process when aeration is restarted after removal of the water and solids. After repeated cycles of water removal operation the entire aerobic biological population is depleted to the point where the aerobic digestion process ceases and the entire contents of the digester tank water and solids, whether fully digested or not, must be removed for final disposal.
Another problem frequently encountered surrounding this shutdown period is the release of very objectionable odors such as hydrogen sulfide and ammonia created by the anaerobic biological activity that takes palace during the extended time the aeration is shut off.